Nondestructive downhole seismic vibrator source and processes of utilizing the vibrator to obtain information about geologic formations

ABSTRACT

The invention relates to a nondestructive downhole seismic source capable of generating S V  -waves, S H  -waves, and P-waves alone or in combination to determine information about a surrounding geologic formation. The invention also includes processes of performing crosswell tomography and reverse vertical seismic profiling. The invention also includes a means and process to carry out in hole seismic logging operations.

This application is a continuation-in-part of application Ser. No.841,074 filed 3-18-86, now U.S. Pat. No. 4,702,343. This application isrelated to application Ser. No. 841,073.

FIELD OF THE INVENTION

This invention relates to downhole seismic sources. More specifically,this invention relates to nondestructive hydraulic downhole seismicsources and processes of obtaining information about undergroundformations penetrated by at least one well bore.

BACKGROUND OF THE INVENTION

During drilling or after the completion of a well, it is often desirableto obtain information about the formation surrounding the well bore. Inwells completed into or through zones in the formation containinghydrocarbons, it is desirable to have techniques of obtaininginformation about these producing zones. Well logs are often run todetermine such parameters as resistivity, conductivity and otherparameters from which the properties of the oil and the producingformation can be deduced to give a clearer picture of the environmentsurrounding the well bore.

One measurement technique involves the use of geophones which arelowered into the well bore while surface seismic sources generate wavesto pass through the geologic formation. The geophones sense these waves.Subsequent processing of the recordings derived from these waves providea clearer picture of the environment surrounding the well bore. However,the weathering layer of the earth's crust attenuates a great deal of theenergy from the seismic sources before it reaches the zones of interestin the formation surrounding the well bore. Providing a seismic sourcewithin a well bore below the weathering layer and spaced from adifferent well bore in which the geophones are located would remove theeffects of the weathering layer.

Typically, a well is completed into a formation by cementing a linerwithin the well bore. Any source used within the well bore must becapable of imparting the desired amount of energy into the formation tobe detected either at the surface or in an adjacent well bore. However,the form of the energy must not input shear or compressive boreholestress capable of separating the liner from the cement or causingdamage. These stresses should be less than the maximum recommended shearstress on the casing cement interface, which is about 20 psi, asspecified by the American Petroleum Institute (API RP2A Oct. 22, 1984).

FIG. 12 and FIG. 13 are graphs comparing the borehole stresses andeffective energies between known destructive downhole sources, namely, a1.1-lb (500 gm) dynamite charge, a 40-cu. in. (655 cc) airgun, and a4,000 lbf output (18,000 newtons) seismic source of the invention. Thevalues are applicable around 100 Hz, a frequency which is within theusual frequency band. The figures show the seismic vibratory sourceachieves high effective energies with low borehole stresses. The 10 psiborehole stress induced by the seismic source of the invention is onlyhalf of the recommended maximum induced shear stress on the casingcement interface according to American Petroleum Institute standards(API RP2A Oct. 22, 1984).

Currently, a technique called "Vertical Seismic Profiling", (VSP), isused to obtain information about particular zones of interest within theformation surrounding the well bore. Typically, this technique requireslowering a geophone into a well bore while providing various surfaceseismic energy sources, such as dynamite, to impart seismic energy intothe formation to be received by the geophone. The geophone is lowered toa specific depth within the well bore and a surface seismic energysource is located on the surface to impart the seismic energy into theformation which is detected by the geophone and recorded. The geophoneis moved to a different depth and the process is repeated. The surfaceseismic energy source is moved to various locations at various offsetsfrom the well bore and the process is repeated. This process isextremely costly and time-consuming. The repeated use of destructivesurface sources renders this process unsuitable for use in populatedareas unpopular with many surface landowners.

A great deal of the time and expense could be eliminated if anondestructive seismic source could be placed in a well bore to be usedin a reverse VSP (RVSP) process. The RVSP process requires a source in awell bore and numerous geophones on the surface. The seismic energygenerated from the seismic source is detected by the geophones on thesurface. Since numerous geophones can be laid out in a two-dimensionalarray from the well bore, the location of the geophones can remainconstant while the seismic source moves instead of requiring movementboth of the seismic source and the geophones.

Assuming a nondestructive seismic source can be located within a wellbore spaced from an adjacent well bore containing one or more geophones,superior crosswell tomography processes of production wells can beperformed because the source can be located below the attenuatingweathering layer of the earth's crust.

In studying the subterranean formations, it would be desirable toreceive and analyze a host of different types of seismic waves, e.g.,P-waves, S_(V) -waves, and S_(H) -waves. Thus, it would be highlydesirable to have an apparatus capable of nondestructively generatingone or more of these types of waves within a well bore for reception bygeophones located either at the surface or in an adjacent well bore todeduce information about the formation. It would also be desirable tohave a source which can nondestructively generate frequencies in excessof 100 Hz below the weathering layer to perform crosswell tomography andcrosswell profiling. Crosswell tomography between adjacent well borescannot be adequately performed at the present time using oil wellsbecause present available downhole seismic sources, e.g., dynamite orthe air gun are all impulsive sources generating shear stress far inexcess of API standards for maximum shear stress in casing-cementinterface. Attempts to tomographically image reservoirs using surfacedata gathering sources, e.g., geophones, have failed due to theattenuation and filtering of the higher frequencies by the weatheringlayer, as well as unfavorable source-receiver configuration.

SUMMARY OF THE INVENTION

I have invented a nondestructive downhole seismic source capable ofgenerating waves which can be detected by sensing devices positioned inthe well bore itself, on the surface or in an adjacent well bore. Thesource can generate P-waves, S_(V) -waves, and S_(H) -waves alone or incombination to perform RVSP, crosswell tomography, and other usefuloperations for determining information about the formation. The seismicsource can also be used to record seismic in-hole logs by recording theseismic signal using a receiver in the same well as the downholevibrator. The receiver should be spaced a distance from the seismicsource clamping means but attached to the seismic source. An attenuatingspacer will be between the source and the receiver to dampen thevibrations traveling along the tool containing the source and receiver.Preferably the nondestructive downhole seismic source generates not onlylower frequencies but also frequencies in excess of 100 Hz and upwardsof 500 Hz or higher. As the frequency of the seismic waves generatedincreases, the resolution of any measurements increases. Thus,properties of a thinner zone within the formation can be analyzed. Thiswould enable the production engineers to develop specific techniques tomaximize the output of hydrocarbons between producing zones and/orextend the life of the producing zones. Additionally, the seismic sourcecan achieve nondestructive output forces of up to 4,000 lbf (18,000newtons) at frequencies up to 500 Hz or higher from a reaction mass ofonly about 300 lbs (136 kg). This power and frequency capability isorders of magnitude greater than heretofore available from existingdownhole vibratory sources such as air driven vibrators or vibratingpackers. Other impulsive sources with high seismic power output havetended to damage the cement-casing bond in the well bore due to the highinduced stresses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of a downhole seismic source within acutaway view of a well bore;

FIG. 2 illustrates an alternative embodiment of a downhole seismicsource within a well bore;

FIG. 3 illustrates a top view of the downhole seismic source of FIG. 2within a well bore;

FIG. 4 illustrates a further alternative embodiment of a downholeseismic source within a cutaway view of a well bore;

FIG. 5 illustrates a top view of the embodiment of FIG. 4;

FIG. 6 illustrates still another alternative embodiment of a downholeseismic source within a cutaway view of a well bore;

FIG. 7 illustrates a graph of the force output of a downhole hydraulicseismic source illustrated in FIG. 2;

FIG. 8 illustrates the generation of seismic S_(V) -waves and P-waves ofa downhole seismic source;

FIG. 9 illustrates an alternative seismic wave generated by the downholeseismic source;

FIG. 10 illustrates three-dimensional reverse vertical seismic profilingusing the downhole seismic source; and

FIG. 11 illustrates crosswell tomography using a downhole seismicsource.

FIG. 12 is a graph comparing borehole stresses between downhole sourcesof dynamite, airgun, and 4,000 lbf seismic source of the invention; and

FIG. 13 is a graph comparing effective energies between downhole sourcesof dynamite, airgun and vibrator source of the invention.

FIG. 14 illustrates the radial reaction mass as well as the apparatusused to orient the reaction mass in the proper direction.

DETAILED DESCRIPTION OF THE INVENTION

The various process and apparatus embodiments of the invention will bemore clearly illustrated by referring to the Figures. FIG. 1 illustratesa downhole seismic source 10 within an earth formation 100 penetrated bya well bore 500. The seismic source 10 is lowered into the well bore 500with a cable 29 attached to a suitable anchoring means 28.

The seismic source 10 includes an upper body 12 incorporating a suitableclamping means to retain the source securely in place at a predeterminedlocation within the well bore. The clamping means should generate enoughforce to interlock the source 10 within the casing or well bore withoutbreaking well bore, casing, or liner or damaging the cement. Theclamping means should generate a force which is at least greater thanthe force output generated by the seismic source during operation. Thisis usually about twice the generated vibrating force but in any caseless than the force necessary to fracture or break the casing or thewell bore liner or damage the cement. A suitable clamping means in thesource 10 is illustrated as a hydraulic clamping means comprising thehydraulic pistons 14a and 14b connected together by a plate 16. Duringthe operation of the clamping means, a hydraulic line 18, which is partof cable 29, is pressurized from the surface which forces the pistons14a and 14b out of their liner and engages plate 16 with the well bore500.

By accelerating a reaction mass contained in a clamped borehole tool inone radial direction, one obtains a shear displacement perpendicular tothe direction of the generated force and a compressional displacementparallel to the force. The reaction mass can be accelerated using highpressure hydraulic fluid and the piston arrangements illustrated in FIG.14.

A fluid supply line 700, supplies high pressure fluid from the pumplocated at the surface or in other areas of the tool. A clamping means701 similar to that described in the above embodiments is used to clampthe tool in the well bore 706. Reaction mass 702 is oriented to movegenerally perpendicular to the well bore axis. Servovalve 703 controlsthe flow of fluid into and out of a cylinder 704. Hydraulic fluid returnline 705 is also provided.

The two different displacements, compressional and shear, are separatedby 90° but occur simultaneously. The magnitude of the shear displacementis approximately three times that of the compressional displacement.This will result in a horizontally polarized shear wave radiatedperpendicular to the direction of the motion of the reaction mass and aradially polarized compressional wave radiated parallel to the motion ofthe reaction mass. The radiated shear wave will not have the rotationalpolarization the rotating reaction mass (described below), which can beadvantageous in several applications.

In order to control the direction of the radiated S- and P-wave energy,the direction or orientation of the downhole tool needs to bedetermined. This can be done using a downhole gyro 710 of the typeGyrodata, Inc., Houston, Tex., supplies. To rotate the radially movingreaction mass in order to radiate the S- or P-wave energy in a desireddirection a separate gear mechanism 707 is provided which allows theseparate rotation of different sections of the tool.

The gyro is used to determine the orientation of the tool in the wellbore, and the gear mechanism is then used to rotate the reaction massportion of the tool such that it is in a desired orientation. This may,for example, be useful when the well bore is deviated.

A hydraulic actuator means 22 is attached to the body 12 by any suitablemeans such as bolts, welds and the like. The piston 26 in the actuator22 is capable of moving a reaction mass 24 attached thereto parallelwith the well bore to generate an S_(V) -seismic waves in the horizontaldirection illustrated in FIG. 8 and also P-waves from the vertical tosub-vertical (0°-45° from vertical). The descriptive directions (e.g.,horizontal, vertical) apply to the typical vertically oriented well. Themovement of the reaction mass 24 to generate seismic waves isillustrated as 1000. The hydraulic actuator 22 is powered by hydraulicfluid from the surface through line 20. The size of mass and length ofstroke are a function of the desired amplitude and frequency of theseismic wave to be generated.

It is anticipated that actuator 22 could be adapted to be capable ofmoving a reaction mass 24 attached thereto in a motion perpendicular tothe well bore to generate horizontal compressional P-waves (asillustrated in FIG. 9) and vertical shear waves S_(V) -waves) from thevertical to sub-vertical (0°-45° from vertical). The movement of thereaction mass 24 to generate seismic waves is illustrated in FIG. 9 as2000). FIG. 14 provides further details of the source when the reactionmass is oriented to have its motion perpendicular to the well bore.

Alternatively, apparatus 10 can be operated so as to generate a seismicwave by the engaging clamping plate 16 with the well bore 500 throughthe pressurizing of the pistons 14a and 14b. The varying of clampingforces exerted by the pistons 14a and 14b in a swept frequency modegenerates horizontal compressional P-waves (as illustrated in FIG. 9)and vertical shear waves (S_(V) -waves) from the vertical tosub-vertical (0°-45° from vertical). The movement of the pistons togenerate the P-waves is illustrated as 2000. Though seismic waves can begenerated by impacting the well bore 500 with clamping plate 16, thepreferred method has the plate 16 contacting the well bore 500 at aforce necessary to maintain the source at a fixed position. Thereafter,additional pressure is exerted on the well bore through plate 16 in apulsating, constant or oscillatory manner so as to generate the desiredseismic wave into the formation 100. Of course, the strength andmagnitude of the seismic wave 2000 is limited by the desire of notfracturing the well bore casing or formation.

FIGS. 2 and 3 illustrate an alternative seismic source 30. The source 30contains a housing or body 32 which includes two hydraulically actuatedpistons 34a and 34b attached to a clamping plate 36 for positioning thesource 30 within a well bore 500. In this embodiment, an electricalmotor driving a hydraulic pump illustrated as 38 is in communicationwith pistons 34a and 34b. A signal from the surface, passing through anelectrical line 54 in the cable 58, causes the valve in the hydraulicpump 38 to move hydraulic pistons 34a and 34b. The movement of thepistons 34a and 34b forces the clamping means 36 into communication withthe well bore 500. A suitable electric motor and hydraulic pump means isavailable from Vickers Inc., Jackson, Miss., or Moog, Inc., Buffalo,N.Y. Any pump motor combination which can operate at 100° F. andgenerate 3,000 psi pressure or higher at a 0.5-5 gpm flow of hydraulicfluid is suitable in the invention.

The body 32 further contains a motion sensing device 50 in communicationwith surface electronics so that the movement of the device within thewell bore during operation can be measured. Standard accelerometers usedin well bore environments are suitable for this application.

Connected to the base of the body 32 is an electrically controlledhydraulic actuator 40 capable of moving a reaction mass 46 through themovement of a hydraulically driven piston 44 in a linear accelerator 42.A suitable source for a hydraulic actuator linear acceleratorcombination is manufactured by the Moog Corporation, Buffalo, N.Y. Thisactuator-accelerator combination is capable of imparting a wide range ofvarying frequencies to reaction mass 46. These are controlled from thesurface by an electronic signal sent through wire 56. The wires 54 and56 are electrical portions of a cable 58 which has sufficient strengthto lower and raise the source 30 within the well bore 500. Cable 58 isattached to the source 30 at cable anchor means 52.

The reaction mass 46 should additionally provide for a space to containa motion sensing device 48 (such as an accelerometer) to provide a baseline measurement for the frequencies generated during operation.

A top plan view of the source 30 positioned in the well bore isillustrated in FIG. 3. The edges of the body 32 have serrated orcrenelated engaging portions 32a and 32b to position the body 32securely within a well bore 500 when clamping plate 36 is pressurizedvia piston 34a and 34b. The clamping plate 36 may also have a serratedsurface 36a. The serrations 36a, 32a and 32b generate very high pointforce when clamping plate 36 contacts the well bore 500 therebyeliminating or minimize slippage, effectively making the clamp amechanically stiff device.

The embodiment illustrated as 30 in FIG. 2 possesses the additionalbenefits of not requiring hydraulic lines from the surface. Source 30 iselectrically powered from the surface and has a self-contained hydraulicpower supply means necessary to reciprocate the reaction mass 46 andengage the clamping means with the well bore. This embodiment avoids thevery high pressure losses encountered in the long hydraulic lines when asource is positioned in a deep well and the hydraulic power is providedfrom the surface. By shortening the hydraulics, i.e., making theapparatus self-contained, fluid friction losses which occur in longhydraulic lines are avoided and higher forces at higher frequencies canbe generated by reciprocating the reaction mass 46 and moving the plate36. The embodiment of FIG. 1 having hydraulic lines from the surface mayprove more convenient for shallow applications.

One reaction force frequency range for 25-lb (11 kg) and 75-lb (34 kg)reaction masses are illustrated in FIG. 7 for the apparatus described inFIGS. 2 and 3. The curves represent the vibration of the reaction massesparallel with the well bore 500 to generate S-waves illustrated by themovement 1000 in FIG. 2.

FIGS. 4 and 5 illustrates a preferred embodiment of the invention. Thedownhole seismic source is illustrated as 60. The source 60 has a body62 with four clamping plates 66a, 66b, 66c, and 66d, which aresymmetrically and radially positioned out from the source. The plates66a-d are engaged to meet the well bore 500 through pistons 64a-hthereby forming a means for clamping source 60 securely to the well bore500. The clamping means allows transfers of seismic S_(V), S_(H) andradial P-waves from source 60 into the formation. In a preferredpulsating mode, the pistons 64a-h may be simultaneously pulsatedradially, varying the force applied to the well bore 500, generatingseismic waves out into the formation. Of course, the source 60 can bepreferably designed with only three clamps if it proves advantageous tocentralize through triangulation. An advantage of this device is that itcan generate radial seismic waves by oscillating the force applied tothe formation through the plates in an omni-directional fashion to avoidthe problems of unidirectional seismic wave generation. The devicehaving one or more clamps can operate to perform the processes of thepresent invention.

The device functions as follows: A signal is sent from the surface viacable 58 to an electric motor 67 which powers a hydraulic pump 68. Aservo control valve 69 (servovalve) actuates the pistons 64a-h viahydraulic line 72 and the piston actuator valve 74. The piston actuatorvalve 74 also controls the piston 64a-h functions which generate radialseismic waves through the pulsating of opposite pairs of pistons. Theservovalve 69 directs the hydraulic fluid to either the verticalactuator 70 or rotational actuator 71 connected to the reaction mass 73.A motion sensing device 50 (such as an accelerometer) monitors theoutput of the seismic source 60. Suitable sources for the electric motorand the hydraulic pump combination include Reda Pump, Bartlesville,Okla., and Vickers, Inc. The servovalve, and actuators along with thehydraulic pistons are available from Moog, Inc.

The downhole seismic source 60 also includes motion sensing devices 79which are secured to the well bore wall 500 by expandable boot 78.Suitable sensing devices 79 include geophones or accelerometers designedto detect vibration of the borehole wall 500. The sensing devices 79 areisolated from the rest of the seismic source by an acoustic isolator 75but connected to the device 60 by electric wires 76 and hose 76a forinflating the boot 78. The wires 76 terminate in a dewar module 77 whichcontains the electronics sensing devices 79. A suitable source for theexpandable boot 78 is Baker Tool of Houston, Tex.

The other motion sensing devices 50 included within the seismic source60 are not isolated from the seismic source generating portion of theapparatus. By comparing signals from the isolated sensing devices 79 andthe nonisolated devices 50, information may be obtained on the clampingeffectiveness and the energy transmission into the formation. Anelectronics package 77 receives command signals from the surface andcontrols the various components of the seismic source. The electronicspackage also receives signals from the motion sensing devices 79 and 50and transmits them to the surface. It is preferred that the electronicspackage 77 be isolated from the vibrations of actuator 60 by theacoustic isolator 75 to avoid vibration damage to sensitive electronicinstruments. Further vibrational protection may be achieved bypositioning the electronics package 77 below the boot 78 and motionsensing devices 79.

In this preferred embodiment, the source 60 is held in place by aseparate clamping means which is distinct from the clamping means in theprevious embodiments. The separation of the seismic source generationand the clamping function into distinct units permits the secureclamping of the seismic source 60 within the well bore 500 without thepotential slipping of the seismic source 60 as the seismic waves aregenerated. A secure clamping action is generated by the metal to metalcontact of a driving wedge 82 connected to a hydraulically actuatedtwo-way piston 81 within the body portion 80 of the clamping means. Asthe driving wedge 82 moves down, it forces contact wedges 83a and 83binto the well bore 500 when driving wedge 82 moves up, it retractscontact wedges 83a and b from the well bore 500. The metal to metalcontact of the driving wedge 82 with the contact wedges 83a and 83bcreates a more secure clamping action than hydraulic fluid displacementwithin a piston cylinder. Contact wedges 83a and 83b may also be springloaded so they retract into the source 60 when not securing the source60 within the well bore 500 (helpful, for example, if the toolexperiences a power failure. Of course, this wedge clamping means can beincorporated into any of the other embodiments if more secure clampingis desired.

The wedge clamp design of FIG. 4 may be particularly preferred when themotion of actuator vibration is in the same direction as the clampactuators as in the case of a perpendicularly oriented actuator orreaction mass. The fluid-filled pistons of the clamp actuators act as ashock absorber which distort or absorb vibrational energy intended to betransmitted into the formation. A driving mechanism such as hydraulicpiston 81 or a mechanical screwdrive (not shown) drives a driving wedge82 parallel to the borehole 500 which in turn drives contact wedges 83aand 83b which are dovetail to the driving wedge 82 out into contact withthe well bore 500. Such a wedge clamp design does not depend solely onhydraulic pressure in the direction of vibratory motion to secure thetool.

This preferred embodiment operates according to the following process.The device 60 of the invention will be able to generate radial,torsional, or vertical forces, one at a time. The reaction mass 73serves as a load to both the vertical 70 and the rotational (torsional)71 servoactuators. To achieve the size limitation, the reaction mass maybe constructed of a heavy material such as Kennertium(®). A 300-lb (136kg) reaction mass of Kennertium(®) would be approximately 3.5 in. (9 cm)in diameter and 45 in. (114 cm) long. When activated, the verticalservoactuator 70 above the mass 73 drives the mass 73 are reactedthrough the device housing 62, through the clamp 64/66 and to the wellbore wall 500. The same reaction mass 73 is attached to the rotaryservoactuator 71. When activated, the rotary servoactuator 71 drives thereaction mass 73 torsionally.

Radial forces are preferably generated independent of the reaction mass.Linear servoactuators operating simultaneously and acting directly onthe well pipe wall generate the radial forces. These actuators arelocated below the mass and act radially and are spaced symmetricallyapart from each other. As illustrated in FIG. 5, four clamps 64/66 arespaced at 90° from each other. A three-clamp version (not shown) wouldhave clamps spaced at 120°. These orientations provide multi-directionalforce output.

Each axis of the force generation requires appropriate electronics todrive the corresponding servovalve and provide closed loop servoactuatorcontrol for that axis. The vertical servoactuator is proposed as aposition control servo. A linear variable displacement transducer (LVDTposition transducer) located at the bottom of the reaction mass providesmass position feedback for the axial servo. A sinusoidal positioncommand to the vertical servoactuator will result in mass position,velocity and acceleration waveforms that are also sinusoidal.

The rotary servoactuator is also proposed as a position control servo. Arotary LVDT position transducer located at the bottom of the reactionmass provides angular position feedback for the rotary servo actuator.Prior to cycling the rotary axis a constant position command is appliedto the vertical position loop to hold the mass at some nominal positionoff the axial end stops. A sinusoidal angular position command to therotary servoactuator will then result in mass angular position, velocityand acceleration wave forms that are also sinusoidal. The sinusoidalacceleration is reacted at the well pipe wall through the clamp with atorque equal to the product of the mass rotational inertia and theangular acceleration.

The radial servoactuators are proposed as a pressure control servo. Asingle three-way operating servovalve drives the control side of each ofthe three actuators. A pressure transducer sensing this common controlpressure provides pressure feedback for the radial servoactuator. Priorto cycling the radial axis a nominal pressure command is applied to loadthe radial actuators via pads against the well pipe wall. A sinusoidalpressure command signal superimposed on the nominal loading commandsignal will result in a sinusoidally varying force applied radially tothe well pipe wall. The absolute force at each of the actuator pads isequal to the product of the control pressure and the net area of eachactuator. The radial actuators are retracted by means of a spring returnattached to the center of each of the pads. For a sinusoidal positioncommand signal the mass position will be given by χ_(m) SIN(ωt). χ_(m)is the peak mass displacement and ω is the radian frequency ofvibration. The mass velocity will be given by χ_(m) (ω₂)COS(ωt). Themass acceleration will be -χ_(m) (ω₂)SIN(ω t). The resulting outputforce is then

    F=m(χ.sub.m)(ω.sup.2)SIN(ωt.

Limits on maximum force output as a function of frequency are imposed bylimited stroke, supply pressure, servovalve flow and servovalvedynamics.

The rotational axis torque (force) generator uses a position servo asdescribed previously. For a sinusoidal angular position command signalthe mass angular position will be given by θ_(m) SIN(ωt). θ_(m) is thepeak mass angular displacement and ω is the radian frequency ofvibration. The mass angular velocity will be given by θ_(m) (ω)COS(ωt).The mass angular acceleration will be -θ_(m) (ω²)SIN(ωt). The resultingoutput torque is then

    T=J(θ.sub.m)(ω.sup.2)SIN(ωt)

where J is the rotational inertia of the reaction mass about thevertical axis. The resulting torsional force at the pipe wall is givenby

    F=T/r

where r is the inside radius of the well pipe wall.

Primary limits on maximum torque (force) output as a function offrequency are imposed by limited stroke, supply pressure, servovalveflow and servovalve dynamics.

The radial axis force generator is a pressure control servo as describedpreviously. A nominal static pressure command signal results in acontrol pressure (P_(o)) common to all three actuators. A sinusoidalpressure command signal added to the nominal command gives a commonoutput pressure of P_(o) +P_(c) SIN (ωt). P_(c) is the peak outputpressure change from nominal (P_(o)) and ω is the radian frequency ofvibration. The resulting force at each of the three interfaces of radialactuator paid and pipe wall is then

    F=P.sub.o (A)+P.sub.c (A)SIN(ωt)

where A is the net area of each radial actuator.

Limits on maximum radial force output as a function of frequency areimposed by supply pressure, servovalve flow and servovalve dynamics.Transfer of radial force to the pipe wall is very direct and notdependent upon a path of force transfer through the clamp.

It is anticipated that the actuator of the invention is capable ofgenerating forces between 1,000 and 18,000 newtons (220-4,000 lbf) andeven higher. A preferred level of power for many applications would befrom 4,000-18,000 newtons for applications requiring higher power suchas a deep VSP survey. Generating forces above 18,000 newtons will placestresses on the casing cement near the maximum allowed. Cement logsshould be examined in all cases, particularly when generating forcesnear 18,000 newtons and above, to ensure the cement is not damaged.

FIG. 6 illustrates a further alternative embodiment of a nondestructivedownhole seismic source 90. The seismic source 90 contains a housing 91including a clamping means (92a and 92b and 93) driven to engage thewell bore 500 within the formation 100 through hydraulic pistons 92a and92b. A hydraulic pump 88 powers the pistons 92a and 92b and a servovalve95 which oscillates a reaction mass 97 through a piston 96 to form aS_(H) -wave. The torsional movement of the piston 96 to generate theS_(H) -wave is illustrated as 3000.

Of course, any of the above-identified embodiments to generate S_(V)-waves, radial P-waves, S_(H) -waves, and P-waves can be incorporatedamong the various embodiments to satisfy particular applicationsrequired for a downhole hydraulic source.

FIG. 8 illustrates both graphically and pictorially the operation of adownhole source 30 of the invention (such as source 10 of FIG. 1, source30 of FIG. 2, source 60 of FIG. 4, or source 90 of FIG. 6) within a wellbore to generate seismic waves. A logging truck 200 containing a cable300 suspends source 30 into a well bore 500 through an appropriate rig400. Graphic pictorial illustration demonstrates the radial S_(V) -wavesand P-wave components of a reaction mass operating reciprocatinglywithin the well bore illustrated as 1000. The movement of the reactionmass generates S_(V) -waves and a weaker P-wave component.

FIG. 9 illustrates pictorially and graphically the P-wave and S-wavecomponents of a downhole source 30 of the invention (such as source 10of FIG. 1, source 30 of FIG. 2, source 60 of FIG. 4, or source 90 ofFIG. 6) when operated so as to generate a predominantly P-wave in thehorizontal direction (in relation to a vertical well bore). The reactionforces generated by the source 30 are perpendicular to the well bore.The movement is illustrated as 2000 in FIG. 9. The horizontal P-wavecomponent is substantially larger than in FIG. 8.

FIG. 10 pictorially illustrates a cutaway three-dimensional view of anRVSP operation using a downhole source 30 of the invention (such assource 10 of FIG. 1, source 30 of FIG. 2, source 60 of FIG. 4, or source90 of FIG. 6) RVSP can be easily and quickly accomplished with morebeneficial results than VSP through the use of the downhole seismicsource 30 to generate the seismic wave with a multitude of surfacegeophones 675 illustrated as strings of geophones 650a-d connected to arecording truck 600. The logging truck 200 lowers the seismic source 30which is operated to generate the seismic waves of interest, i.e., S_(V)-waves, P-waves or S_(H) -waves, while the geophones record the directand reflected seismic waves (1000/2000). Since the delicate geophones donot have to be lowered into the well bore environment and a vast arrayof surface geophones that can be utilized, the profiling of theformation can be done more quickly and efficiently than in VSP.

FIG. 11 illustrates the use of a downhole source 30 of the invention(such as source 10 of FIG. 1, source 30 of FIG. 2, source 60 of FIG. 4,or source-90 of FIG. 6) to perform crosswell tomography. In crosswelltomography, a first well 400a containing well bore 500a contains atleast one or a plurality of seismic sources, e.g., 30, which generatethe desired seismic waves, i.e., S_(V) -waves, S_(H) -waves, or P-waves,directed out into the formation toward at least one receiver well 400bcontaining well bore 500b.

The well 400b contains geophones 650a-n suspended therein but connectedto the appropriate recording truck, not illustrated, on the surface. Asthe seismic source or sources 30 in well 400a generate seismic waves,these waves are recorded by receivers 650a-n within the well bore 400bto generate a seismic picture of the geologic formation between wells400a and 400b. Since the weathering layer normally attenuates seismicwaves generated on the surface, surface tomography cannot be carried outfor frequencies greater than about 100 Hz due to the attenuationproblems. Of course, the lower the frequency, the less refined thereadings are because the greater the wavelength of the seismic waves andthus the lower the resolution. Using a downhole seismic source enablesthe generation of high frequency seismic waves, i.e., greater than 100Hz, to raise the resolution so that smaller and smaller anomalous zoneswithin a given formation can be identified. This can be extremelyvaluable for applications where the producing zones are relatively thinand thus information about them is hard to obtain.

The force output hydraulic pumps and actuators exceed the force usingexisting pneumatic actuators by a factor of 100 and the force output byheretofore existing electromagnetic actuator by a factor of 20-40. Thefrequency output of the hydraulic actuators is between 10-1500 Hz. Themaximum frequency of the pneumatic vibrator is approximately 100-200 Hz.The difference in frequency output is due to the vast difference in thestiffness between a gas and a fluid.

The motion sensing devices 79 shown in FIG. 4 also enable the presentinvention to operate as a downhole seismic logging tool. The seismicvibrations generated by the actuator 73 travel into the formation 100where they are reflected or refracted back and then detected by thesensing devices 79. By analyzing the signals detected, information aboutthe formation surrounding the well bore may be deduced. Most surfaceseismic recording is in a range of 10-200 Hz. The downhole vibrator ofthe present invention may generate seismic signals at the same frequencyas that recorded from surface seismic sources. Logs obtained by thepresent invention using the same frequency as surface generated seismicdata will provide better correlation with surface generated seismic datathan that obtained by existing logging tools which operate at muchhigher frequencies. The present invention also has greater wavepenetration than existing tools because of its higher power outputcapability.

Three component geophones are preferred for the motion sensors 79.Though FIG. 4 illustrates only a single motion sensor package (79), aseries of separately coupled sensor packages is envisioned. Each sensorpackage would be acoustically isolated from both the other seismicsensor packages and the seismic source housing 62.

The invention has been described with reference to particularlypreferred processes of using the downhole seismic source andparticularly preferred embodiments of the hydraulic down seismicvibrator source. Modifications which would be obvious to one of ordinaryskill in the art are contemplated to be within the scope of theinvention.

What is claimed is:
 1. A downhole seismic source comprising:a housing;means for clamping the housing in a well bore; a reaction mass in saidhousing, said reaction mass oriented to move generally perpendicular tothe well bore axis; means for hydraulically moving said reaction masssuch that forces are transmitted through, through said clamp and intothe well bore to generate at least one detectable seismic wave ofpredetermined frequencies to said housing; and means for rotating saidreaction mass around an axis generally parallel to the well bore axis soas to orientate said mass in a selected direction.
 2. The source asrecited in claim 1 further comprising a means for determining anorientation of said source.
 3. The source as recited in claim 2 whereinsaid means for determining an orientation is a downhole gyro.